Natural gas is one of the most promising alternative fuels because it is abundant and extremely clean burning. According to the U.S. Department of Energy, exhaust emissions from natural gas vehicles (NGVs) are much lower than those of gasoline-powered vehicles. For instance, carbon monoxide (CO) and nitrogen oxides (NOx) are reduced by more than 90 and 60 percent, respectively, and carbon dioxide (CO2), a greenhouse gas, is reduced by 30 to 40 percent. When used in medium- and heavy-duty engines, CO and particulate matter (PM) reductions of over 90 percent, and NOx reductions of over 50 percent, have been demonstrated compared to diesel engines. However, one of the reasons that NGVs have not been as widely accepted as gasoline-powered vehicles is the lack of an extensive public refueling infrastructure.
Natural gas is typically stored aboard a vehicle either as a gas (e.g., compressed natural gas) or as a liquid (e.g., liquefied natural gas (LNG)). When natural gas is stored as a gas, it can be in the form of a compressed natural gas (CNG) or an adsorbed natural gas (ANG). CNG typically refers to natural gas stored in a tank without any adsorbent and compressed to a relatively high pressure (such as about 20 MPa (i.e., about 200 bars) or higher). ANG typically refers to natural gas stored in a tank containing a natural gas adsorption material and compressed to a relatively low pressure (such as about 3.5 MPa (i.e., about 500 psi) or lower).